The HFP01 Self-Calibrating Heat Flux Sensor is primarily intended for high accuracy heat flux measurement studies within the soil medium it is buried. The sensor incorporates a passive thermopile detector embedded within a thin ceramic plastic composite body, with an integrated thin film heating element mounted atop the sensors composite body. The HFP01 outputs a linear millivolt signal generated by the differential temperature across the sensors body and proportionate to the local soil heat flux. The heating element can be cycled on/off at user defined timing intervals to perform an in-soil calibration check, thus allowing the user to derive sensor calibration factors ideally matched to the soil conditions at the time of measurement; implicitly cable connection, data acquisition and data processing are also tested. For certain application environments, the user may wish to consider the installation of two HFP01 sensors; sensor redundancy offers the possibility of improved spatial averaging and QA of the processed measurement result.